1. Field of the Invention
This invention relates to power amplifiers for electric signals in the audio frequency range.
2. Description of the Related Art
Power amplification is an important requirement in many different areas of the audio industry, such as high quality equipment used for monitoring during mastering and mixdown, large and powerful sound reinforcement systems used for live performances, and consumer level audio reproduction equipment. Improvements in recording and reproduction technology have placed increasingly heavier demands upon audio amplifiers to deliver more power, provide greater dynamic headroom and generate less distortion. New high quality loudspeaker systems can be particularly sensitive to amplifier deficiencies.
Most present power amplifier designs use the voltage feedback approach made popular by the availability of modern integrated circuit operational amplifiers. These designs are relatively inexpensive and can provide adequate amplification levels. However, they do have some disadvantages. The bandwidth of a voltage feedback operational amplifier is generally inversely proportional to its closed-loop gain; this can lead to a serious degradation in performance at the higher gain levels. Also, the slew-rate of voltage feedback amplifiers is usually restricted because the transconductance stage has a finite maximum output current available to charge the compensation capacitor. This can lead to poor dynamic intermodulation distortion performance. Small compensation capacitor values can be used to increase the slew-rate. This, however, requires a degeneration of the input stage to reduce the transconductance in the interest of amplifier stability, which in turn reduces the amplifier's open-loop gain. The reduction in loop gain available in the audio band in turn leads to an increase in the closed-loop amplifier distortion, most of which originates with the highly non-linear output stage. An audio amplifier designer is thus faced with a dilemma, because a trade-off must be made between stability, open-loop gain and slew-rate without compromising AC performance and transient response.
One type of amplifier that does not exhibit the bandwidth variation of voltage feedback amplifiers is the current feedback operational amplifier. While this amplifier displays some variation in bandwidth as the gain is increased from unity to moderate values, the variation is much less significant than with voltage feedback amplifiers. Current feedback amplifiers do not begin to exhibit the bandwidth variation of voltage-feedback amplifiers until the closed-loop gain is made quite large. Also, current feedback operational amplifiers almost generally have higher slew rates than voltage-feedback amplifiers for a given quiescent supply current, and exhibit relatively low transient distortion.
The basic architecture of a conventional current feedback amplifier is shown in FIG. 1. An input buffer 2, implemented with transistors Q1-Q4, forms a voltage follower which forces the inverting input to the potential of the non-inverting input. The collectors of transistors Q3 and Q4 supply reference currents to respective current mirrors 4 and 6, whose outputs are applied to a gain node 8. Any imbalances in the collector currents of Q3 and Q4 are reflected by the current mirrors and summed at the gain node, charging a capacitor C1. This voltage developed across C1 in turn is applied to an output terminal 10 through an output current gain stage consisting of transistors Q9-Q12.
Negative feedback is provided from the output terminal 10 to the inverting input of input buffer 2 through a feedback resistor R.sub.FB. This feedback circuit tends to counteract current imbalances in the current mirrors, thus causing the output voltage at terminal 10 to track the input signal at the non-inverting input to input buffer 2. Resistors can be added from the inverting amplifier input to ground to increase the gain, in the same fashion as for a conventional operational amplifier.
Although the current feedback amplifier achieves a high large-signal bandwidth and slew-rate as discussed above, it also has certain disadvantages. In particular, the low frequency or DC characteristics are not ideal. With both current feedback and voltage feedback operational amplifiers, there is an input voltage offset associated with mismatches between the transistors in the input stage, and this results in a finite output offset voltage. The offset voltage can be made acceptably low by conventional techniques such as resistor trimming to obtain precise values. However, there is another component of the output voltage offset which is present only with current feedback amplifiers, and not with voltage feedback designs. This component of offset results from a bias current which originates primarily from imbalances between the two current mirrors, and is usually larger than the offset voltage associated with the input buffer. Current feedback amplifiers are thus not normally used at low frequencies, significantly below 1 MHz, and in particular their advantages have not previously been applied to audio frequency power amplifiers.